1. Field of the Invention
The present invention relates to a method according to the preamble of the appended claim 1 for attaching a micromechanical microphone. The invention relates also to a micromechanical microphone attached according to the method.
2. Description of the Related Art
The efficacy of receiving acoustic signals is primarily determined by the conversion performance of a microfone between acoustic and e.g. electrical energy. The distortion and frequency response of the microphone is, in turn, significant with respect to sound quality. In several audio applications, the aim is to optimize for instance microphones in such a way that sound quality, costs, the size of the device, producibility and other productive aspects result in an acceptable device unit.
Frequently for instance microphones place restrictions on the application. One impediment, for example, for reducing the dimensions of mobile phones is the physical size of the microphone. The microphones currently known are structurally separate, encapsulated components which are coupled by means of connector pins or the like, arranged in the housing of the microphone, either directly to a circuit board or electrically to other circuitry by means of separate connection wires or springs. In microphones, the signal conversion is based on a transformation, i.e. more generally, on a change in the mutual geometry between two transducer means, such as a diaphragm and a back plate. In microphones, the transformation is produced with sound. At least one transducer means is elastically transformable, e.g. flexible or compressible. Consequently, the microphones are composed of several discrete components, while the internal integration level of the component remains fairly low.
It is possible to divide microphones into different types according to the operational principle. The microphone types most commonly used in acoustics are based on an electrostatic or electromagnetic (a moving coil or magnet) principle, or to the piezoelectric phenomenon.
In electrostatic microphones, for example two, advantageously planar diaphragms or plates, placed in the vicinity of each other and forming a capacitor, can be used as transducer means. The first diaphragm is typically elastic or flexible, and the second diaphragm is made stationary. The transformation is based on the alteration in the capacitance between the transducer means, which is an outcome of a change in the distance between the diaphragms. The force between the diaphragms depends, for instance, on electric charges present in the diaphragms, and on other mechanical structures.
In microphones, sound generates deformations in an acoustic means, which deformations are coupled into an electric signal according to the physical principles presented above. For example, a capacitor microphone is provided with an electrically conductive diaphragm, which vibrates with the sound. An electrically conductive back plate is typically placed parallel to the diaphragm, wherein the diaphragm and the back plate form a capacitor which has a capacitance value defined by its geometry. Because the deformation produced by sound, i.e. a deflection in the diaphragm, alters the distance between the diaphragm and the back plate, the capacitance of the capacitor changes accordingly.
To detect an alteration in the capacitance, an electric potential difference is arranged between the diaphragm and the back plate, and the diaphragm and the back plate are coupled to an amplifier circuit, for example to the gate of a JFET transistor in a way known as such. The potential difference can be formed, for example, with a bias voltage, wherein a direct voltage is conducted between the diaphragm and the back plate. Instead of the bias voltage, it is also possible to use a prepolarized electret material combined either to the back plate and/or to the diaphragm, wherein the microphone is called an electret microphone. Consequently, the change in the capacitance creates a varying voltage signal which can be amplified in a conventional amplifier. Thus, in this microphone type, the first transducer means is the diaphragm and the second transducer means consists of the back plate.
In the piezoelectric phenomenon, the stress state of an object releases charges from the material and, inversely, charges conducted into the object generate stress states. In such a microphone, the first transducer means is an object in which the piezoelectric phenomenon occurs. The substrate of the first means, with respect to which the first means is deformed, can be used as the second transducer means. The force between the transducer means depends, for example, on the material used, the dimensions, the voltage generated, and on other mechanical structures.
By means of micromechanics, it is possible to produce small-sized components, such as microphones and pressure transducers. In micromechanical components, silicon is typically used as a substrate. The production takes place either subtractively or additively. In subtractive production, silicon is chemically discharged from predetermined points on a silicon wafer, wherein a desired micromechanical component is produced. In additive production, a so-called additive method is used, wherein desired layers are added on a suitable substrate. In the production of micromehanical components, it is possible to use both of these methods. In micromechanical components, the thickness of the layers is typically in the order of micrometers. In addition to various silicon compounds, it is possible to utilize for instance metallization to produce e.g. conductors.
A micromechanical microphone typically comprises a diaphragm and a back electrode, between which there is an air gap whose thickness is typically in the order of 1 .mu.m. Furthermore, the micromechanical microphone typically comprises a back chamber, with which it is possible to affect, for instance, the frequency response of the micromechanical microphone. The height and volume of this back chamber is typically many times the air gap between the diaphragm and the back electrode respective the volume between them. FIG. 1 presents the structure of such a micromechanical microphone of prior art in a reduced cross-section.
In micromechanical microphones, the back electrode is typically perforated, wherein in a stable situation, the pressure on both sides of the back electrode is substantially equal. Furthermore, a venting system for pressure balancing is typically arranged from the back chamber or directly through the pressurized diaphragm, wherein the pressure of the back chamber will be substantially equal to the stable air pressure prevalent in the environment of the micromechanical microphone.
The volume of the back chamber, i.e. the so-called back volume is a substantial factor in microphone design when setting the acoustic properties of the microphone. The acoustic properties desired for the microphone depend, for instance, on the use of the microphone. For example in telephone use, a smaller band-width will be sufficient than in microphones intended for HiFi applications. Another criterion for microphone design is the sensitivity of the microphone, i.e. the smallest pressure fluctuation the microphone reacts to. A further criterion is the noise of the microphone itself, which in micromechanical microphones is caused by thermal vibrations in the diaphragm and thermal noise from both conductors and semiconductors.
U.S. Pat. No. 4,922,471 discloses another micromechanical microphone. This microphone is formed of two silicon chips, provided with a diaphragm in between them. The back electrode is formed as an inflexible structure, and at the same time it forms the back chamber. Furthermore, the back electrode is provided with a FET transistor, whereby the microphone signal is amplified.
Moreover, according to prior art, micromechanical microphones are encapsulated to facilitate the handling of microphones in connection with storage, transportation and attachment to the end product. The connection leads of the microphone are connected to connector pins formed in the housing, or they are formed as separate conductors through the housing. One reason for the encapsulation of the micromechanical microphone is the fact that this is a better way to ensure that the geometry between different functional parts of the micromechanical microphone remains as good as possible all the way to the end product.
Micromechanical microphones of prior art which comprise housings and other structures are, however, relatively large compared with the micromechanical microphone as such. This is due to, for instance, the fact that in the end product the micromechanical microphone is, first of all, inside a housing of its own, and further, this encapsulated microphone is inside the housing of the end product. Furthermore, the the size of the micromechanical microphone is increased by the fact that the micromechanical microphone is typically electrically coupled to the rest of the electronics of the device by means of leads.
One drawback complicating the use of acoustic transducers of prior art is the space they require due to, for instance, the fact that the first transducer means and the second transducer means have to be encapsulated, and the transducer has to be constructed separately to be mechanically rigid. Thus, the space required by the housing increases the need of space for the acoustic transducer. These factors restrict especially the reduction in the size of portable devices. Furthermore, encapsulation raises the price of acoustic transducers.